The invention in general relates to the field of chemical analysis, more particularly to nuclear magnetic resonance spectroscopy (NMR) and high-pressure liquid chromatography (HPLC), and still more particularly to a dual-function NMR probe capable of functioning in alternative modes with either a stationary-sample vessel (test tube) or a flow cell.
An NMR apparatus is most often characterized in gross by cylindrical symmetry. A typical NMR magnet is of the superconducting variety and is housed in a dewar which includes a room temperature cylindrical bore in which a very carefully controlled homogeneous magnetic field is sustained by operation of the superconducting magnet in the interior of the dewar. An NMR probe holds a sample placed in the uniform magnetic field. The housing for the probe is typically cylindrical to fit within the bore of the magnet and the sample is generally positioned along the central (longitudinal) axis of the probe. A coil is disposed close to the sample within the probe to apply radio frequency (RF) pulses to the sample. The resultant resonance signal of the sample is picked up by the coil and delivered to measurement electronics. The measurement electronics generate an output signal, and take the Fourier transform of the signal to obtain an NMR spectrum.
NMR spectroscopy has been used with both flow-through and stationary samples. In flow-through NMR, measurements are run as the sample flows through a sample cell. Flow-through NMR is particularly useful when coupled to a separation technique such as high-pressure liquid chromatography. In stationary-sample NMR, the sample is usually placed in a closed test tube, and measurements are performed while the sample remains in the test tube.
Typical NMR probes are customized for use with either flow cells or test tubes. An end user is not typically able to use the same NMR probe with both flow cells and test tubes. While test tubes are easily replaced in conventional systems, conventional NMR flow cell assemblies, including the NMR sample flow cell together with its various connectors and associated tubing for attachment to an HPLC, are delicate, difficult to handle and not well suited for removal or insertion in the field. Removal and insertion of such assemblies in the NMR probe are risky and expensive, at least in part because the flow cells and attached connectors are positioned and secured to the NMR probe within nested assemblies of coils, dewars, and support structures. Many present designs require significant mechanical interaction with these closely mated subassemblies. Electrical manipulations are often needed to exchange the flow cell, such as unsoldering and resoldering of the RF and pulsed field gradient coils. Some designs have RF circuitry directly attached and secured to the flow cells. There is an additional cost and risk associated with exchange of the flow cell in these designs because of the directly secured RF circuitry. Moreover, some manufacturers void a system""s warranty if the end user removes the NMR probe housing. As a result, an end user who needs to run both stationary-sample and flow measurements typically uses a separate NMR probe for each measurement type.
The present invention provides NMR probes, systems, kits and methods allowing the use of a single NMR probe with both flow cells and stationary-sample vessels. The present invention allows an end user in the field to conveniently and quickly convert a probe between flow and stationary-sample configurations, without removing the probe""s housing, RF coils, electrical connections, or other sensitive components.
The present invention provides a dual-function nuclear magnetic resonance (NMR) probe comprising a radio-frequency (RF) coil, an upper insulator held in fixed position above the coil, a lower insulator held in fixed position below the coil, and a guide tube held in fixed position below the lower insulator. The upper insulator has an upper longitudinal sample-holding aperture for sequentially centering a stationary-sample vessel and a flow cell in the radio-frequency coil. The upper sample-holding aperture has a tapered guiding section for guiding the stationary-sample vessel from above through the upper insulator. The lower insulator has a lower longitudinal sample-holding aperture aligned with the upper sample-holding aperture, for centering the flow cell within the radio-frequency coil. The guide tube serves to guide the flow cell from below through the lower sample-holding aperture. The stationary-sample vessel and the flow cell are present in the probe sequentially (alternatively).
In the preferred embodiment, the upper insulator comprises an outer part having a longitudinal inner bore, and a guiding/pressing ring mounted within the inner bore. The guiding ring defines the tapered guiding section of the upper sample-holding aperture. The tapered guiding section is preferably formed by a chamfer at an upper edge of the guiding ring, although generally the tapered guiding section can be situated within the guiding/pressing ring. The guiding/pressing ring further serves to flexibly press longitudinally on an insert coupled to the RF coil, for reducing the vibration or other undesired motion of the insert and RF coil. The guiding/pressing ring comprising an outer contact section for engaging the outer part of the upper insulator, an inner contact section for engaging the probe insert, and a longitudinally-flexible intermediate section flexibly connecting the outer section and the inner section of the ring.
At least two longitudinal flow-connection tube apertures formed in the outer part of the upper insulator serve to accommodate a flow connection tube through the upper insulator. The flow connection tube passes through one of the flow-connection tube apertures when the probe is in a flow configuration, and through both flow-connection tube apertures when the probe is in a stationary-sample configuration. In the stationary-sample configuration, the flow connection tube extends out from one of the flow connection tube apertures and into another of the flow connection tube apertures on an external side of the insulator.
In an alternative embodiment, to allow the use of the probe with flow cells and stationary-sample vessels of different transverse sizes, different centering rings are provided for insertion in the upper insulator in the stationary-sample and flow configurations. In the stationary-sample configuration, the upper insulator is formed by an outer part having a longitudinal inner bore, and a stationary-sample vessel centering ring positioned within the inner bore. The stationary-sample vessel centering ring centers the stationary-sample vessel in the radio-frequency coil. In the flow configuration, the upper insulator is formed by the outer part and a flow cell centering ring positioned within the inner bore. The flow cell centering ring centers the flow cell in the radio-frequency coil. The two rings are provided as part of a kit for conveniently converting the NMR probe between its stationary-sample and flow configuration.